Integrated Circuits (IC) surround us today and are present in almost every device, apparatus or accessory. Generally IC's consist of a variety of electronic components constructed as a single piece of semiconductor, built up in layers, manufactured in a process called “photolithography”.
In photolithography a semiconductor wafer (usually made from silicon) is first coated with a photoresist (light-sensitive) layer. Then, the coated wafer is exposed to a predetermined pattern of light that is acquired by passing light (laser or lamp light) through a reticle (sometimes called “mask”)—a blank, usually made form fused silica (quartz) or silicon dioxide, having an engraved pattern on it. Lastly, the irradiated wafer undergoes chemical development—washing off the wafer to remove the exposed (or unexposed) coating, depending on positive or negative photoresist type, and etching cuts through the wafer's uncoated areas.
Photo masks used for integrated circuits (IC) manufacturing are commonly composed of a chrome layer deposited on quartz or fused silica plates, which are subsequently patterned for a photolithographic process.
In a photolithographic process, ultraviolet (UV) light passes through the patterned Chrome, and an image is formed within the Photoresist layer on top of a Silicone wafer.
It is essential that the chrome layer of the mask be defect free, with no voids, pinholes or scratches, so that there are no parasitic defects printed onto the Photoresist layer of the wafer in the photolithograpic process, for defects in the chrome layer of the photomask inevitably and ultimately result in defects in the produced IC wafer.
The chrome layer is occasionally accompanied by additional layers, such as protective layers, antireflective layers etc.
IC manufacturing steps include two different major phases— Front line or front-end, and back-end. The first phase includes all printing steps on Silicone wafers, and the latter includes the final integration into the chip package, in particular for “flip-chip” technology.
Front-end processes require sub-micron resolution, typically with an optical demagnification factor of 1:4, where the back-end masks resolving power is normally more then 10 times less (above few microns) with optical magnification of 1:1 for the printing of conductive “bumps”.
Additional applications similar to IC back-end masks are the lithographic processes for LCD (liquid crystal displays) or FPD (flat panel displays), and the thin film magnetic read/write heads used in disk drives for personal computers.
Non-uniformity in some mask steps, such as with contact-holes diameters, is also a widely spread defect, which needs to be addressed by a repair mechanism.
A structure of pixels inside the transparent quartz mask, with a correct design, can ultimately control the amount of UV energy transmitted by mask contact-holes and Vias, to the required level.
One well-known mask repair technology used for front-end masks is the Ion beam deposition (mostly with Gallium or Carbon ion sources).
However, such a system is highly complex, equipment cost is high, and problems of Gallium ions stains and Quartz damage are unavoidable.
Therefore it is not suitable for the back-end masks repair, and cost to performance ratio is questionable for front-end masks.
A second repair method, first published in 1998 (R.Haight et al: SPIE 3546, 477 (1998)) and 1999 (Haight et al: MARS: Femtosecond laser mask advanced repair system in manufacturing. Journal of vacuum science & technology B, November/December 1999, p. 3137) featured a Femtosecond pulsed laser system, used at an ablation mode, for direct Chrome removal only.
In Such a system, laser beam is directed above the Chrome coating side, and the glass substrate is not processed. No scanning system is provided for the laser beam, and it is limited in speed, hence cannot generate complex 3D patterns, for it consists of only a 3 axis moving stage for positioning. Only Chrome ablation process is performed.
Economical mask repair technologies for back-end masks are currently not available, or too complex to be implemented.
For front-end mask repair, Ion-beam is sometimes used, however, there are no simple and low-cost systems which can handle Chrome voids, pinholes etc.